Section 112. The structure of the retina demands fuller notice. Figure 9 shows an enlarged, diagram of a small portion of this, the percipient part of the eye. The optic nerve (o.n. in Figure 8) enters the eye at a spot called the blind spot (B.S.), and the nerve fibres spread thence over the inner retinal surface. From this layer of nerve fibres (o.n. in Figure 9) threads run outward, through certain clear and granular layers, to an outermost stratum of little rods (r.) and fusiform bodies called cones (c.), lying side by side. The whole of the retina consists of quite transparent matter, and it is this outermost layer of rods and cones (r. and c.) that receives and records the visual impression. This turning of the recipient ends away from the light is hardly what one would at first expect-- it seems such a roundabout arrangement-- but it obtains in all vertebrata, and it is a striking point of comparison with the ordinary invertebrate eye. Section 113. We may pause to call the student's attention to a little point in the physiology of nerves, very happily illustrated here. The function of a nerve fibre is the conduction of impressions pure and simple; the light radiates through the fibrous layer of the retina without producing the slightest impression, and at the blind spot, where the rods and cones are absent, and the nerve fibres are gathered together, no visual impressions are recorded. If there is any doubt as to the existence of a blind spot in the retinal picture, the proof is easy. Let the reader shut his left eye, and regard these two asterisks, fixing his gaze intently upon the left-hand one of them. * * At a distance of three or four inches from the paper, both spots will be focussed on his retina, the left one in the centre of vision, and the right one at some spot internal to this, and he will see them both distinctly. Now, if he withdraws his head slowly, the right spot will of course appear to approach the left, and at a distance of ten or twelve inches it will, in its approach, pass over the blind spot and vanish, to reappear as he continues to move his head away from the paper. The function of nerve fibres is simply conduction, and the nature of the impressions they convey is entirely determined by the nature of their distal and proximal terminations.